I. Proteins Modifications by Activated Carcinogens. A) Interaction of RNase with the carcinogen, N-acetoxy-3-FAA, yielded 3 modified proteins distinguishable from RNase by a change in elution properties upon ion exchange chromatography. The modification was the result of the transfer of the acetyl group from N-acetoxy-3-FAA to RNase. B) Modification of histones of calf thymus and rat-liver nuclei by N-acetoxy-2-FAA. Interaction of histones of calf-thymus with the carcinogens resulted in acetylation as well as arylamidation of fraction F1 to an approximately equal extent. Histones F2 and F3 were modified predominantly by acetylation. Epsilon-N-Acetyl-L-lysine was isolated from as enzymatic hydrolysate of F1, F2 and F3. The histones of rat-liver incubated with N-acetoxy-2-FAA were also modified by acetylation. Acetylation predominated over arylamidation in unresolved as well as resolved histone fractions. The isolation of epsilon-N-acetyl-L-lysine from the histones of rat-liver nuclei is in progress. II. N-Hydroxylation of the Carcinogen 2-FAA by hepatic microsomes of the rat and guinea pig. Previous studies on the N-hydroxylation of 2-FAA by hepatic microsomes of the rat, a species susceptible to the action of 2-FAA, are being extended to the guinea pig, a species refractory to the action of 2-FAA. The correlation between the extent of N-hydroxylation of N-arylamides and the magnitude of the spectral shift obtained on addition of these compounds to microsomes is under study.